The present invention relates to component design and, in particular, to a method and system for producing verified component designs and to assist in tool design to near-production specification in the materials of choice, via production processes and in considerably shorter time scales than has previously been possible.
Traditionally, pattern makers and tool makers have undertaken key roles in bringing new component designs through design evaluation, development and modification to production standards. Until comparatively recently, such work was labour-intensive, time consuming and, as a result, costly. More recently, computer aided design has enabled some of the early evaluation steps to be carried out before a new design is reduced to a 3-D prototype. Nevertheless, a point is inevitably reached during design evaluation when a 3-D prototype of the new design is required.
So-called xe2x80x9crapid prototypingxe2x80x9d techniques have been developed which enable designs to be produced in 3-D form using a variety of techniques now well established in the art. Such rapid prototyping methods allow compression of timescale in the production of a component master pattern.
The drawback of current rapid prototyping methods is that the 3-D representation which results is not necessarily made from the production material of choice and, in any case, is not made via a production process. For example, one known rapid prototyping method is so-called xe2x80x9claminated object manufacturingxe2x80x9d in which the computer design is recreated in 3-D form as a multiplicity of laminated layers. A component master pattern is produced in a material, such as paper, which is easily laid down as thin layers bonded together. However, some form of tool then needs to be made to produce a mould for replicating the design in the correct material using a production process. The rapidly-produced prototype or simulant material part is only of limited use in the evaluation process because the material from which it has been produced and/or the method by which it has been produced are not the same as the material and/or method that will be used in full scale production.
Despite its limitations, if evaluation of the rapidly-produced prototype is favourable, the conventional methods of producing a mould tool, using traditional tool room methods or, alternatively, using sintered or resin based materials, must then be employed to take the verification process forward. As discussed above, these known methods are costly and time-consuming to implement, and have their own particular limitations regarding parameters such as temperature, geometry, pressure and surface finish.
It is also true that the time and cost penalties of changing component design and hence tooling when modifications are required will influence the majority of designers to follow a well-defined, minimal risk path based on their experience. This means that they tend to be inhibited about deviating from conventional techniques.
What is therefore needed is an intermediate verification step which enables a quickly-produced component to be obtained from a master pattern, in the material of the designer""s choice and using a production process. Preferably, this design verification step should be carried out as early as possible during the design process, for example at the concept stage or as soon as a master pattern can be made.
It is therefore an object of the present invention to provide a method of producing designs in the material of choice with relative ease, relatively quickly and cost effectively compared to conventional methods. It is another object of the invention to provide a method of producing designs via a production process. It is a further object of the invention to enable verification of component design to be carried out prior to commitment to high cost tooling upon finalisation of a design. It is a still further object of the invention to provide a process which enables design iteration to be carried out relatively easily and cheaply, thereby giving both engineers and designers greater design freedom before commitment and with a hitherto unattainable degree of confidence that the resulting production parts will satisfy the design criteria.
The invention is a method of producing an article in a mould cavity formed by casting around a pattern of the article to be produced, the method comprising:
(a) pouring a mould matrix material around said pattern in a mould bounded by solid retaining means;
(b) causing said mould matrix material to harden to produce a flexible moulding medium having the following physical properties:
flexural strength in the range 20-300 MPa;
flexural modulus in the range 700-10,000 MPa;
tensile strength in the range 16-200 MPa;
tensile modulus in the range 850-10,000 MPa;
compressive strength in the range 24-500 MPa;
compressive modulus in the range 400-10,000 MPa;
hardness in the range 5-100 Vickers; and
relative density in the range 1-10
(c) removing the pattern from the flexible moulding medium to leave a mould cavity conforming to the profile of the pattern;
(d) forming or moulding an article in the mould cavity under production-representative conditions of temperature and pressure, and
(e) removing the article from the mould cavity.
Advantageously the flexible moulding medium has a flexural strength in the range 30 to 100 MPa and preferably around 40 MPa.
Advantageously, the flexible moulding medium has a flexural modulus in the range 1000 to 4000 MPa and preferably around 1500 MPa.
Advantageously, the flexible moulding medium has a tensile strength in the range 20 to 70 MPa and preferably around 22 MPa.
Advantageously, the flexible moulding medium has a tensile modulus in the range 1000 to 4000 MPa and preferably around 1300 MPa.
Advantageously, the flexible moulding medium has a compressive strength in the range 30 to 120 MPa and preferably around 40 MPa.
Advantageously, the flexible moulding medium has a compressive modulus in the range 600 to 2000 MPa and preferably around 1000 MPa.
Advantageously, the flexible moulding medium has a hardness in the range 7 to 80 Vickers and preferably around 9.5 Vickers.
Advantageously, the flexible moulding medium has a density in the range 1.2 to 5 and preferably around 1.3.
Preferably, the pourable material is a curable resin such as a urethane polymer cured by incubation for a short spell (about one hour) at room temperature in the presence of an isocyanate cross-linking agent. Most preferably, the pourable material is a polyester-based polyurethane.
The pourable material may be loaded with a variety of fillers to regulate the properties of the hardened material which forms the flexible mould. For example, the pourable material may include suspended particulate metal to improve the heat transfer characteristics of the cured mould. Alternatively, a material, such as glass or ceramic beads, could be added to impart better insulation capacity. Similarly, additives can be incorporated to influence hardness, rigidity, toughness, operating temperature range and such like in the cured mould.
The exact nature of the physical additives will vary according to the particular additive material in question. For example, in the case of particulate metal additives, the buoyancy of the additive particles relative to the matrix material must be taken into consideration. A buoyancy approaching neutrality is best, otherwise there may occur marked settlement of the added particulate material during hardening or cure of the matrix material. A certain degree of settlement is permissible and may even be desirable, for example in the preparation of a mould which needs to have its thermal conductivity boosted for moulding hot materials. If metal particles gravitate towards the split line during mould cure, thermal conductivity enhancement is greatest in the portion of the mould immediately surrounding the mould cavity. This makes the mould more tolerant of hot moulded product.
Generally, the fillers or additives are included in an amount ranging from 30 to 70% in proportions by volume measured relative to the total volume of mould matrix material. At proportions below 30% by volume, the additives tend to lose their effectiveness and the mould might as well be formed using undiluted matrix material. At proportions greater than 70% by volume, the additives tend to dominate the physical properties of the mould matrix material and some of the advantages of using a dynamic material are lost. In particular, the bond lengths formed in the cured material are relatively shorter and the cured matrix material therefore loses some of its rubber-like qualities. Also, the higher the filler content, the more difficult the material becomes to handle in its uncured state. For example, high filler contents mean that the material may be unsuitable for manual mixing.
Preferably, the additives are included in an amount ranging from 40 to 60% in proportions by volume, more particularly in an amount ranging from 45 to 55% by volume, especially 50% by volume.
Typical non-conductive additives include talc, Molochite (Registered trade mark)xe2x80x94an alumino-silicate refractory material proprietary to English China Clay International, and glass. Typical particle sizes are 200 microns and below, and it will be understood by persons skilled in the art that additive particles should have an even granule size to encourage homogeneity in the mould during curing.
One of the primary functions of the filler material is to combat shrinkage in the mould matrix material as it cools. It is important that the cured mould matrix material is thermally stable in the sense that it has dimensional stability over its working temperature range. Typically, the unhardened mould material is capable of being poured over a temperature range of 0xc2x0 C. to 200xc2x0 C. and, once hardened, is able to accept a working range of moulding materials having melt flow temperatures varying between xe2x88x9240xc2x0 C. and 600xc2x0 C. At the upper limit of this working range, it is important to minimise the length of time for which the mould matrix material is exposed to elevated temperature, otherwise the mould matrix material may become permanently degraded to the detriment of moulding fidelity in the finished component. Therefore, it is advisable in such circumstances to load the mould matrix material with a conductive filler, such as steel particles, to distribute the thermal energy of the moulding material quickly through the matrix material.
Mild steel particles may be used as a filler for non-corrosive moulding materials, but stainless steel particles are preferred if the moulding material is in any way corrosive. For example, many rubber compositions include a high sulphur content which renders them highly corrosive. Stainless steel particles would therefore be recommended for moulding components from rubbers.
Fibre fillers could also be used, but these cause a marked increase in viscosity in the unhardened mould matrix material. As a result, the unhardened material may become unworkable manually, requiring mechanical mixing apparatus. Also, the pouring properties are altered, so it becomes vital to optimise the rate of hardening after pouring to ensure that the mould matrix material assumes the desired form around the component master pattern before hardening is complete.
Besides fillers, which alter mould properties by physical means, it is also possible to influence the properties of the cured mould by chemical means, by varying the chemical formulation, such as changing the nature of the pre-polymer or using a different blend of starting materials.
It is a key feature of the hardened mould matrix material that it possesses an elastic memory over the quoted operating temperature range. The elastic memory is defined at the time the material is poured against the pattern and caused to hardenxe2x80x94this sets the memory to the shape of the component master pattern. When the mould form is distorted during the moulding process, for example as a result of the applied injection pressure, it has dynamic power to return to its original shape when the applied pressures in the mould are released.
It can be seen from the foregoing that it is relatively easy to change the flexible medium to suit particular criteria and, in particular, to match the moulding requirements of a particular end product.
One of the key advantages of using a pourable material to form the mould cavity around a component master pattern is that the features of the pattern are faithfully reproduced, including surface finishes. Moreover, the flexible nature of the cured mould means that undercut formations on the component master pattern are not problematic: the pattern can be jumped from the cured mould with relative ease and the mould reverts to its unstressed form by virtue of its resilience. The same is true for moulded articles subsequently formed in the mould cavity vacated by the component master pattern.
It is also a clear advantage of the present invention that mould formation is so quick and faithful to the prototype, compared to conventional tool making methods, because minor changes to the tool configuration can be accommodated quickly and cheaply. Faithful reproduction of the component master pattern in the cured mould means that draft angles and fillets do not have to be incorporated at every stage, but can be introduced later during the design evaluation process when the exact fillet and draft angle requirements become fully evident.
Another advantage of the present invention is that it can be regulated to give flash-free moulding. In a conventional mould, the applied pressure acts only along the axis of the mould parts and there is a tendency for the material that is being moulded to creep along any lines of weakness, such as along the split lines which are generally oriented perpendicularly to the mould pressing axis. Increasing the moulding pressure may exaggerate the creep problem and it is therefore an acquired skill to judge what moulding conditions will be best for a particular product and/or material to minimise flash yet achieve good product integrity, using conventional moulding techniques.
By contrast, the cured mould material used in the present invention is a flexible form which is capable of exerting equal pressures around the entire orientation of the mould cavity. Hence, an increase in the injection pressure is transmitted into the body of mould matrix material and results in an increase in the mating forces experienced between the mould halves at the split lines. Creep is thereby inhibited and flash-free products result. This is only possible because the mould matrix material is a dynamic material and remains flexible under the moulding pressures applied in the inventive process.
Persons skilled in the art will recognise that, in conventional moulding technology, increasing the injection pressure is likely to cause separation between the mould halves and increase the incidence of flash. The present invention therefore operates in completely the opposite sense from prior art teaching.
Other advantages and modifications of the invention will be apparent to persons skilled in the art from the present description.